Stopoever at Istanbul airport 16th November

Hi people in Istanbul. i live in Barcelona and on a flight back to Barcelona from Bahrain I go via Istanbul and have a stopover of total 6 hours. I would love to see a bit of Istanbul if possible. Any suggestions? What should I see. Should I just grap a taxi? Any other options? I arrive at 8.40 and fly again at 14.00. thanks a lot.